Percussion circuit



Filed Aug. 7, 1961 United States Patent 3,169,694 PERCUSSION CIRCUITHerbert E. Meinema, Lake Forest, ilL, assignor to Harnmond (l rganCompany, Chicago, Ill., a corporation of Delaware Filed Aug. 7, 1961,Ser. No. 129,368 7 Claims. (Cl. 84-112) This application relatesgenerally to musical instruments and more particularly to an improvedpercussion circuit for use therewith.

The use of percussion circuits in electrical musical instruments such asorgans is universal, and the circuits have taken widely different forms.It has been suggested to provide a variable impedance in the form ofelectronic amplifier gain control to provide percussion in circuits suchas that shown in United States Patent No. 2,828,659, issued April 1,1958, to John M. Hanert. These devices, however, are inherentlydistortion producing devices; and therefore proper care in circuitdesign must be utilized in order to achieve a commercially acceptablepercussion circuit, the cornmercial counterpart of the patented cir cuitbeing designed with a minimum of cost to provide an unusually desirablemusical impression with respect to the percussion feature.

It has also been suggested to control the desired percussive envelope bymeans of photoelectric devices in which a photocell is cont-rolled by asource of light to produce a voltage which is a function of the lightintensity for varying the gain of an amplifier. A typical example ofthis approach is shown in United States Patent No. 2,513,109, issuedJune 27, 1950, to Alexander Roth; however, so far as is known, none ofthese photosensitive voltage producing devices has been completelysatisfactory for commercial use in percussion circuits.

Accordingly, it is a primary object of the present invention to providein an electrical musical instrument an improved commercially practicablepercussion circuit characterized by an inherently non-distortionproducing device which is reliable, stable, and eiiicient in operation,and which is very rugged and easily controllable. This object isachieved in the preferred embodiment by the incorporation in the musicaltone channel of a photoresistive device, preferably a cadmium selenidephotorcsistive cell although a lead sulfide or cadmium sulfidephotoresistive cell can be used, and by providing variable intensitylight source for changing the instantaneous resistance of thephotoresistive device in accordance with a pattern producing the desiredpercussion envelope.

It is another object to provide, in one form of the invention, for thecontrol of the variable intensity light source by the tone signals uponthe actuation of a playing key.

It is another object of the present invention to provide, in apercussion circuit of the type called for in the preceding objects,means suppressing the high frequency tone signals which, due to theinherent capacity of the photoresistive device, are passed atappreciable signal levels by the device when its capacitive reactancebecomes lower than the resistance as occurs when the light intensitydecreases to very low values.

It is another object of the present invention to provide an improvedcircuit controlling the attack and decay characteristics of thepercussion envelope and maintaining a low-intensity minimum current flowthrough a neon tube light source at all times in order to minimize theinherent time delay of the photoresistive device in increasing itsresistance to the proper value when the intensity of the light impingingon the device is reduced to its minimum value.

Other objects and the various features of the invention will be apparentupon a perusal of the following descrip- "ice tion taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a diagram, partly diagrammatic and partly schematic, of oneform of the invention; and

FIG. 2 is a fragmentary schematic diagram of a second form of theinvention.

In FIG. 1, a source of musical tone signals 1% is connected by way of abus 11 to the input of a transistor amplifier 12. The source of musicalsignals may be any well known type such as a tone wheel generator inwhich suitably shaped magnetic wheels are rotated adjacent respectivecoils to induce signals, corresponding to the tones of the musicalscale, in the coils. The signals induced in the coils are synthesized ina well known manner by means of playing key switches 13 and a bus barsystem (not shown), the output of which is applied to the bus ll. Sincethe tone wheel generators are inherently low impedance devices, they arefrequently connected to suitable signal amplifying equipment by means ofimpedance matching transformers. However, in the preferred embodiment ofthe present application, signals are fed to the transistor amplifier 12which amplifies the low impedance input without introducing an excessivenoise level.

The transistor amplifier 12 has a low value input resistor 16 connectedbetween the emitter and ground. A parallel connected resistor 17 andcapacitor 2% are connected between the base of the transistor 12 andground. Resistor 18 connects the base of transistor 12 to one end ofresistor The other end of resistor 22 is connected to the positive250-volt anode supply terminal. Resistor it? connects the collector oftransistor 12 to the junction of resistors 18 and 22. The collector isalso coupled to a voltage amplifier 3@ by means of a coupling capacitor32 and a grid leak resistor 33.

The amplifier 36 includes a triode having its cathode connected toground by means of a resistor 34 and its plate connected to a positiveZSO-volt potential by means of a load resistor 36. The plate isconnected to the grid of a triode amplifier so by a coupling capacitor42 and a grid leak resistor 44. The cathode of the amplifier isconnected to ground by a resistor 46, and the plate is connected to apositive 250-volt potential by a resistor 48.

The grid of the amplifier 5% is connected to a photoresistive device 54and thence to an output circuit includinga power amplifier 5t and aspeaker 51 which are illus trated diagrammatically. An attenuatingresistance 53 is connected across the output of the photoresistivedevice 54 and ground. The value of the resistor 53 is twice that of theminimum resistance of the device 54. A capacitor 56 couples the outputof the photoresistive device 54 to the plate of the amplifier 46) toprovide an out of phase current which cancels the current flowingthrough the capacitive reactance of the device 54.

' Capacitor 52, is connected to the resistor 58 which is part of thevoltage divider network comprising resistors 58 and 59. Diode 70connects the junction of resistors 58 and 59 to ground. The diodepolarity is such that when the junction of resistors 58 and 59 isnegative with respect to ground, it is in the conductive state. A glowlamp 60 which by light intensity controls the resistance of device 54 isenergized by a control circuit which includes triode 64, resistors 58,59, 72, 74, 76 and 78, capacitors 63, 66 and diodes 7t), '71. Resistor72 connects the junction of resistors 58, 59 and diode 70 to capacitor66. Capacitor63 connects the junction of resistor 72 and capacitor 66'toground. Capacitor 66 conmeets the junction of resistor 72 and capacitor63 to the grid of triode 64. The resistor 74 connects the junction ofcapacitor 66 and grid of triode 64 to a negative voltage source Thepotential of the negative voltage source is sufficient to cut off theplate current of triode 64 when the circuit is in the quiescent state,i.e.

when no playing key is pressed. Diode 71 is shunted across resistor 74with the polarity such that it is conductive when the junction ofcapacitor 66, grid of triode 64 and resistor 74 is negative with respectto the negative voltage source. Resistor 78 connects the plate of triode64 to ground. The value of this resistor 78 is a function of the desiredratio of peak light intensity to minimum light intensity of the glowlamp. Glow lamp 60 is connected to the junction of the anode of triode64 and resistor 78. Resistor 76 connects glow lamp 60 to the anodesupply voltage.

The operation of the circuit of FIG. 1 will now be discussed. It will beassumed by way of example that the circuit of FIG. 1 is connected in anorgan in a manner similar to that shown in United States Patent No.2,953,055, issued September 20, 1960, to John M. Hanert, and that theoutput of a desired tone wheel generator is coupled to the bus 11 upondepression of one of the playing keys. The tone signal is amplified bythe circuits 12 and 30 and applied to the photoresistive device 54 bycapacitor 42. The tone signal is further amplified by triode 40 andapplied to the voltage divider comprising resistors 58 and 59 by meansof capacitor 52. The anode current of triode 64 is normally cut off bythe negative grid potential and the light emitted by glow lamp 60 is atits minimum value which is determined by the current flowing throughresistor 76, the glow lamp and resistor 78.

Initially the amplitude of the tone signal applied to amplifier 50 islow due to the large voltage drop across device 54 compared to thatacross load resistance 53.

When a playing key is depressed, the positive half cycles of the tonesignal which are developed across the diode 70 and resistor 59 rapidlycharge capacitor 63 by means of resistor 72. This charge, which at thejunction of resistor 72 and capacitor 63 is positive With respect toground, is applied to the grid of triode 64 by means of capacitor 66.The initial charging potential reduces the negative bias on the grid oftriode 64 causing anode current to flow through the glow lamp 60. Theresulting light intensity falling on photoresistive device 54- causesits internal resistance to rapidly decrease to a value lower than thatof resistor 53 with the result that most of the signal applied to thedevice 54 appears as an output signal across resistor 53 which isapplied to the input of power amplifier 50. After the initial chargepotential causes rapid conduction by the triode 64, a voltage will beginto develop across capacitor 66 at a rate determined by the value ofcapacitance of 66 and resistor 74. This voltage will finally assume avalue such that the negative potential on the grid of tube 64 will beits original negative potential of 3 volts. At this time there will beno current flowing through resistor 74 so the voltage across it will bezero. The time necessary for the potential on the grid to becomeseven-tenths of its final value is known as the time constant of thecircuit, which is essentially the time constant of the decay of thepercussion envelope.

When the key is released, the voltage across resistor 59 and capacitor63 will very rapidly drop to zero. The charge potential across capacitor66 will then appear across the grid of triode 64 and ground. Assumingthat diode 71 were not used, this potential will be 6 volts negative onthe grid, driving the tube further into the anode current cutofi region.If the playing key were suddenly depressed again, the voltage on thegrid would only drop to 3 volts negative and the tube anode currentwould still be cut off with the result that no percussive signal wouldbe produced. The charge on capacitor 66 would drain off throughresistors 59, 72 and 74 at the same rate as the initial charge when thekey is released. The value of resistance 74 is many times that ofresistors 59 and 72 and it is thus the controlling resistance value ofthe charge-discharge rates.

However, it is necessary to have the discharge rate of capacitor 66greatly increased when a key is released so that a percussive tone willimmediately be produced it the key is quickly depressed again. This isaccomplished in the circuit by connecting a diode across resistor 74With the polarity such that the grid of tube 64 can never be morenegative than the negative bias supply voltage. Capacitor 66 is thenforced to discharge through resistors 59 and 72 and diode 71, theresistors being of very low Value compared to resistor 74 greatlyreducing the discharge time of capacitor 66. Thus two percussion tonescan be produced in rapid succession.

Photoresistive devices possess some capacitance and when they areoperated in the near dark region where their resistance is of the orderof many megohms, the capacitive reactance at higher audio frequencies islower than the resistance. This is detrimental to the operation of suchdevices in percussion circuitry as the higher frequencies are notattenuated sufliciently to become inaudible. This photocell capacitivereactance is nullified in the applicants circuit by feeding an out ofphase current through a capacitor to the output terminal of thephotoresistive device 54. If the current due to the capacitive reactanceof device 54 is equal to the out of phase current supplied through theauxiliary capacitor, cancellation of the reactive signal occurs and theoutput signal appearing across resistor 53 is purely a function of theratio of photoresistive device resistance to shunt load resistance. Inthis circuit the out of phase current is obtained from the anode oftriode 40 and is fed through the auxiliary capacitor 56 to the junctionof photoresistive device 54 and load resistance 53. The value ofcapacitance of capacitor 56 is a function of the gain of triode 40 andthe effective capacitance of photoresistive device 54 and can be readilycalculated by anyone skilled in the art.

There is an additional important reason why this photoresistive devicecapacitive reactance should be minimized. In the circuit shown, if thereactive current is not canceled and the original tone signal is keyedduring the part of the cycle which produces transients, the higher orderharmonics so produced will appear directly across the output resistor 53before the percussive tone which is slightly delayed time wise. Thisresults in a noticeable click which is objectionable.

Typical values for certain of the components are as follows:

Resistor 16 10 ohms. Resistor l7 220 ohms. Resistor 18 4700 ohms.Resistor 19 15,000 ohms. Resistor 22 100,000 ohms. Resistor 33 1 megohm.Resistor 34 3300 ohms. Resistor 36 270,000 ohms. Resistor 44 1 megohm.Resistor 46 1200 ohms. Resistor 48 47,000 ohms. Resistor 58 39,000 ohms.Resistor 59 12,000 ohms. Resistor 72 22,000 ohms. Resistor '74 4.7megohms. Resistor 53 100,000 ohms. Resistors 76 and 78 values depend ontype of glow lamp 60 and photoresistive device 54.

reactive signal.

FIG. 2 shows a second embodiment in which a key operated switch ratherthan the tone signals controls the operation of the neon tube controlcircuit. Components corresponding to those shown in FIG. 1 are assignedsimilar numbers.

Thus the tone signals are selected on operation of the playing keys andforwarded through a tube such as 30 to a photocell such as 54 and to apower amplifier 50, as explained for FIG. 1, and a second switch 90 isprovided for each playing key. Actuation of a key operated switch 90connects a positive three-volt potential to the junction of resistor 59and a capacitor 66 whenever a playing key is operated to select aparticular tone. This positive pulse is transmitted through capacitor 66to the grid of triode 64 causing anode current to how through the glowlamp 60. The remainder of the percussion sequence is the same aspreviously described with respect to FIG. 1. That is, the tone signal isapplied to a tube such as 30 and the output extended through a photocellsuch as 54 as disclosed in FIG. 1. In the case of FIG. 2, however, theresistor 58 together with capacitor 52 and diode 76 is omitted, but theglow lamp 60 operates to control the photocell as previously described.

While I have shown and described the preferred embodiments of myinvention, it will be apparent that nu merous variations nadmodifications thereof may be made without departing from the underlyingprinciples of the invention. I therefore desire, by the followingclaims, to include within the scope of the invention all such variationsand modifications by which substantially the results of my invention maybe obtained through the use of substantially the same or equivalentmeans.

I claim:

1. In an electrical musical instrument having a source of musical tonesignals, playing keys for controlling the source to select a desiredtone signal, an output system including a speaker and a tone signalchannel connecting the source to the output system on operation of anyof said keys, a percussion circuit comprising a glow lamp adapted toemit light of one intensity and connected between said source and outputsystem, circuit means effective upon actuation of one of the keys toalter the intensity of the light emitted by the glow lamp at apredetermined rate ad for thereafter automatically enabling said lamp toemit light of said one intensity, and a photoresistive element connectedin the channel and controlled by the light emitted by the glow lamp forcausing the intensity of the signals applied to the speaker to alter incorrespondence with said rate and thereafter return to a conditioncorresponding to light of said one intensity.

2. The combination claimed in claim 1 together with circuit meansenergizing the glow lamp at a desired minimum value when the keys arenot depressed.

3. In an electric organ having a source of musical tone signals, playingkeys for controlling the source to select a desired tone signal, anoutput system including a speaker and a tone signal chanel connectingthe source to the output system on operation of any of said keys, apercussion circuit comprising a glow lamp adapted to emit light of oneintensity and connected between said source and output system, circuitmeans normally energizing the glow lamp at a low current value toprovide light of one intensity, an amplifier for energizing the glowlamp, circuit; means effective upon actuation of one of the keys tocontrol the gain of the amplifier and the in- 6 tensity of the lightemitted by the glow lamp to change at a predetermined rate in a waveformcorresponding to a desired percussion envelope, means for returning thelight intensity of said glow lamp to said one intensity after apredetermined time interval, and a photoresistive element connected inthe channel and controlled by the light intensity emitted by the glowlamp for causing the intensity of the signals applied to the speaker tocorrespond to, the desired percussion envelope whereby said signals arefirst caused to sound loudly and thereafter terminate.

4. In an electric organ of the type having a musical tone generator, keyoperated means for controlling the output of the generator, anelectro-acoustic translating device and circuit means for applying theoutput of the generator to the translating device on operation of saidkey means, the combination with the circuit means of a percussioncircuit comprising a source of light, a photoresistive device connectedin said circuit means and having an electrical resistance characteristicwhich varies in accordance with the intensity of light applied thereto,an amplifier for energizing the source of light, means normally applyinga cutoff bias potential to the amplifier, means including aresistor-capacitor network effective upon the actuation of at leastcertain of the keys for rapidly applying a high positive bias potentialto the amplifier and for gradually returning the bias potential tocutoff to vary the intensity of the source of light in a predetermineddesired waveform, the photoresistive device being controlled by thesource for varying the amplitude of the tone signals in accordance withthe said desired waveform.

5. A percussion circuit for use in an electrical organ wherein any oneof a plurality of playing keys is operated for transmitting respectivetone signals, the improvement comprising an amplifier circuit to whichall of said tone signals are applied, a photoresistive element connectedbetween the output of said amplifier and the input of a secondamplifier, a light control means operated responsive to one of saidtransmitted tone signals for varying the intensity of light interceptedby said element at a predetermined rate and only in response to theoperation of one of said playing keys whereby the resistance value ofsaid photoresistive element is caused to vary at a predetermined ratefor varying the amplitude of the tone signals applied through saidelement to said second amplifier.

6. In the arrangement claimed in claim 5 means for supplying a signalout of phase with said tone signals to said photoresistive element.

7. In the arrangement claimed in claim 5 means for controlling theintensity of said light to reach a predetermined value after apredetermined time period irrespective of the continued transmission ofsaid one transmitted tone signal.

Markowitz June 27,1961 De Miranda et al Feb. 2, 1962

5. A PERCUSSION CIRCUIT FOR USE IN AN ELECTRICAL ORGAN WHEREIN ANY ONEOF A PLURALITY OF PLAYING KEYS IS OPERATED FOR TRANSMITTING RESPECTIVETONE SIGNALS, THE IMPROVEMENT COMPRISING AN AMPLIFIER CIRCUIT TO WHICHALL OF SAID TONE SIGNALS ARE APPLIED, A PHOTORESISTIVE ELEMENT CONNECTEDBETWEEN THE OUTPUT OF SAID AMPLIFIER AND THE INPUT OF A SECONDAMPLIFIER, A LIGHT CONTROL MEANS OPERATED RESPONSIVE TO ONE OF SAIDTRANSMITTED TONE SIGNALS FOR VARYING THE INTENSITY OF LIGHT INTERCEPTEDBY SAID ELEMENT AT A PREDETERMINED RATE AND ONLY IN RESPONSE TO THEOPERATION OF ONE OF SAID PLAYING KEYS WHEREBY THE RESISTANCE VALUE OFSAID PHOTORESISTIVE ELEMENT IS CAUSED TO VARY AT A PREDETERMINED RATEFOR VARYING THE AMPLITUDE OF THE TONE SIGNALS APPLIED THROUGH SAIDELEMENT TO SAID SECOND AMPLIFIER.